How to convert ppm into mass of chemical required

Example Suppose you are asked to prepare an aqueous solution of sodium ions (250.00 mL) of approximate, but accurately known, concentration of 10 ppm from either sodium chloride or anhydrous sodium carbonate.  

Convert the number of moles per litre into moles in the volume required (250.00 mL). Since 1.0 litres of 10 ppm solution of sodium ions requires 0.435 x 10 mol of sodium ions, then 250.00 mL (0.25 L) of solution win need 0.25 x 0.435 x 10 rnol = 0.1087 x 10 mol of sodium ions. 

Calculate the mass of either sodium chloride or sodium carbonate required to maKe up the solution  

This is a term used to describe the effective concentration of a solute. In dilute solutions, solutes can be considered to behave according to ideal (thermodynamic) principles, i.e. they will have an effective concentration equivalent to the actual concentration. However, in concentrated solutions ( 0.5molL ), the behaviour of solutes is often non-ideal, and their effective concentration (activity) will be less than the actual concentration [C]. The ratio between the effective concentration and the actual concentration is called the activity coefficient (y) where 

Equation [6.3] can be used for SI units (molm ), molarity (molL ) or molality (molkg ). In all cases, y is a dimensionless term, since a and [C] are expressed in the same units. The activity coefficient of a solute is effectively unity in dilute solution, decreasing as the solute concentration increases (Table 6.1). At high concentrations of certain ionic solutes, y may increase to become greater than unity. 

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